The present invention relates to mass spectrometry devices and, more particularly, a target illumination device for use in Matrix Assisted Laser Desorption/Ionization mass spectrometry.
Mass spectrometry is a powerful analytical tool in identifying molecular components. Mass spectrometry is a means of identifying these molecular components according to their characteristic xe2x80x9cweightxe2x80x9d or mass-to-charge ratio. Typically, a mass spectrometer includes the following components: an optional device to introduce the sample to be analyzed (this sample is referred to hereinafter as the xe2x80x9canalytexe2x80x9d), such as a liquid or gas chromatograph, direct insertion probe, syringe pump, autosampler, etc.; an ionization source which produces ions from the analyte; an analyzer which separates the ions according to their mass-to-charge ratio; a detector which measures the abundance of the ions; and a data processing system that produces a mass spectrum of the analyte.
Conventionally, various ionization sources, employing various ionization methods, are utilized in order to produce ions from the analyte. One of these ionization methods is referred to as electrospray, in which a sample of the analyte in a solvent is nebulized into aerosol droplets and electric fields induce a charge on the aerosol droplets. The charged aerosol undergoes an ion evaporation process whereby desolvated analyte ions are produced and enter the mass spectrometer for analysis Other conventional ionization techniques include atmospheric pressure chemical ionization and atmospheric pressure photo-ionization.
Each of these ionization techniques is suited to different classes of molecular species. However, for the mass analyzation of macromolecules, including polymer molecules, bio-organic molecules (e.g., peptides, proteins, oligonucleotides, oligosaccharides, DNA, RNA, etc.) and small organisms, e.g., bacteria, the generally preferred method of ionization is matrix-assisted laser desorption ionization (referred to hereinafter as xe2x80x9cMALDIxe2x80x9d). According to the MALDI method of ionization, the analyte is mixed in a solvent with small organic molecules having a strong absorption at a particular laser wavelength (hereinafter referred to as the xe2x80x9cmatrixxe2x80x9d). The solution containing the dissolved analyte and matrix is applied to a metal probe tip or target substrate. As the solvent evaporates, the analyte and matrix co-precipitate out of solution to form a solid solution of the analyte in the matrix on the target substrate. The co-precipitate is then irradiated with a short laser pulse inducing the accumulation of a large amount of energy in the co-precipitate through electronic excitation or molecular vibration of the matrix molecules. The matrix dissipates the energy by desorption, carrying along the analyte into the gaseous phase. During this desorption process, ions are formed by charge transfer between the photo-excited matrix and the analyte.
Conventionally, the MALDI technique of ionization is performed using a time-of-flight analyzer, although other mass analyzers such as an ion trap, an ion cyclotron resonance mass spectrometer and quadrupole time-of-flight are also used. These analyzers, however, must operate under high vacuum, e.g., less than 1xc3x9710xe2x88x925 torr, which, among other disadvantages, may limit sample throughput, reduce resolution and capture efficiency, and make testing samples more difficult and expensive to perform.
To overcome these disadvantages, a technique referred to as atmospheric pressure matrix-assisted laser desorption ionization (hereinafter referred to as xe2x80x9cAP-MALDIxe2x80x9d) has been developed, which employs the MALDI technique of ionization at atmospheric pressure. The MALDI and the AP-MALDI ionization techniques have much in, common, e.g., both techniques are based on the process of pulsed laser beam desorption/ionization of a solid-state target material resulting in production of gas phase analyte molecular ions. However, the AP-MALDI ionization technique does not require the ionization process to occur in a vacuum.
Several apparatus configurations that employ the AP-MALDI ionization technique are illustrated in U.S. Pat. No. 5,965,884 to Laiko (hereinafter referred to as xe2x80x9cthe Laiko patentxe2x80x9d). FIG. 10 (which corresponds to FIG. 1 of the Laiko patent) illustrates an AP-MALDI apparatus having an ionization chamber connected to a spectrometer via an interface. The interface has an inlet orifice into the spectrometer. A sample support extends into the ionization chamber and has a target substrate which is positioned adjacent and orthogonal to a central axis of the inlet orifice. A laser directs a laser beam orthogonally to the target substrate so as to focus on samples deposited on the target substrate, thereby heating and causing the desorption of the sample. The resulting plume of ionized analyte molecules enters the inlet orifice of the spectrometer by virtue of a potential difference established between the target substrate of the sample support and the inlet orifice of the interface.
FIG. 11 (which corresponds to FIG. 5 of the Laiko patent) illustrates another AP-MALDI apparatus which employs a mirror to direct the laser beam onto the target substrate of the sample support. In this AP-MALDI apparatus configuration, the target substrate is orthogonal to the central axis of the inlet orifice of the mass spectrometer. The laser is positioned so that the direction of the laser beam is roughly parallel to the central axis of the inlet orifice. The mirror is positioned to one side of the target substrate and adjacent to the interface, such that it re-directs the laser beam at an acute angle onto a region of the target substrate which faces, and which is directly in front of, the inlet orifice.
However, none of the foregoing techniques produce and collect ions from an AP-MALDI ionization source with satisfactory efficiency. For instance, the apparatus illustrated in FIG. 10 discloses a target which, upon being struck by the laser beam, requires the ionized analyte molecules to travel parallel to the target substrate to reach the inlet orifice, reducing the collection efficiency of the ions. While the apparatus illustrated in FIG. 11 avoids this disadvantage by disclosing an orthogonal arrangement of the target substrate relative to the central axis of the inlet orifice, this arrangement has further disadvantages. For instance, the acute angle at which the laser beam strikes the target substrate does not provide optimal absorption of the laser energy by the target. These, and many other, disadvantages are discussed in greater detail below. Thus, there is a need for an improved method and apparatus for efficiently producing and collecting ions in an AP-MALDI apparatus.
The present invention, in accordance with various embodiments thereof, is directed to an AP-MALDI apparatus for ionizing a target for analysis in a mass analyzer. The apparatus may, according to one embodiment of the present invention, include a chamber, which may be also be called an ionization chamber, that is at or near atmospheric pressure and that contains the target. Pressure in the chamber can also be above or below atmospheric pressure. Thus, it should be understood that, while the term xe2x80x9cAP-MALDIxe2x80x9d is often used herein to refer to the apparatus, the present invention, according to various embodiment thereof, may employ pressures other than atmospheric pressure. The apparatus may also includes a target substrate, which, if an ionization chamber is employed, may be disposed within the ionization chamber. The target substrate has disposed thereon a target e.g., an analyte and corresponding matrix. In addition, the apparatus includes a laser beam produced by a laser, and has an interface between the target and the mass analyzer. The interface has an inlet orifice leading to the mass analyzer. A reflective surface is associated or integral with the interface assembly, and reflects the laser beam toward the target.
According to one example embodiment, the interface is a capillary, and the reflective surface is disposed on an end of the capillary. According to another example embodiment, the interface includes a cap configured to fit on an end of the capillary, and the reflective surface is disposed on the cap. According to still another example embodiment, the end of the capillary is shaped, and the cap is arranged so as to fit on the shaped end of the capillary. According to these and other various embodiments, the present invention enables the AP-MALDI apparatus to maintain the target substrate as close to, and as nearly orthogonal to, a central axis of the inlet orifice as possible, while simultaneously enabling the laser beam to impinge upon the target substrate as nearly orthogonal to the target substrate as possible. In addition, optical leverage is reduced, leading to less critical adjustment tolerances.
The present invention, in accordance pith various embodiments thereof, is also directed to a method for ionizing a target for analysis in a mass analyzer, the mass analyzer having, an interface that defines an inlet orifice of the mass analyzer. The method includes the steps of reflecting, a laser beam off a reflective surface integral with the interface and irradiating the target with the laser beam after reflection. Advantageously, the target is disposed on a target substrate, and the method further includes the step of positioning the target substrate so as to be substantially orthogonal to a central axis of the inlet orifice. In addition, the method may comprise the step of positioning the target substrate with respect to the reflected laser beam in response to a signal from a processor. According to one embodiment of the present invention, the step of irradiating the target includes striking the target with the laser beam such that the laser beam at the target substrate is substantially orthogonal to the target substrate.